The invention relates to a power take-off shaft assembly, particularly but not exclusively for an agricultural tractor, of the kind in which a single power take-off shaft is rotatable at two different speeds and capable of presenting two appropriately different numbers of splines to equipment to be driven thereby. Existing standards require a six-splined power take-off shaft for rotation at 540 revolutions per minute and/or a twenty-one splined power take-off shaft for rotation at 1000 revolutions per minute.